Photovoltaic devices with depleted heterojunctions and shell-passivated nanoparticles

ABSTRACT

Photovoltaic cells are fabricated in which the compositions of the light-absorbing layer and the electron-accepting layer are selected such that at least one side of the junction between these two layers is substantially depleted of charge carriers, i.e., both free electrons and free holes, in the absence of solar illumination. In further aspects of the invention, the light-absorbing layer is comprised of dual-shell passivated quantum dots, each having a quantum dot core with surface anions, an inner shell containing cations to passivate the core surface anions, and an outer shell to passivate the inner shell anions and anions on the core surface.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present invention is a divisional application of U.S. patentapplication Ser. No. 12/890,797, filed Sep. 27, 2010, which claims thebenefit of U.S. Provisional Patent Application No. 61/321,450, filedApr. 6, 2010, and U.S. Provisional Patent Application No. 61/334,650,filed May 14, 2010. The entire contents and disclosure of theseapplications are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention resides in the fields of photovoltaic cells and quantumdots.

2. Description of the Prior Art

Solar cells that generate electricity through the photovoltaic effectrequire a combination of low cost and high efficiency in order for suchcells to offer a practical alternative to traditional means of powergeneration. One way in which the cost of manufacturing a photovoltaiccell can be lowered is by the use of solution processing to form thelayer of light-harvesting material that is part of the cell. Theefficiency of the cell, however, depends on the cell materials,including the light-harvesting material. The optimal light-harvestingmaterial is one that achieves a high short-circuit current densityJ_(sc) by maximizing the absorption of the sun's rays in both thevisible and infrared spectra, and one that extracts a high level ofwork, in the form of a high open-circuit voltage V_(oc) and a high fillfactor FF, from each absorbed photon. For an input solar intensityP_(solar) (typically 100 mW cm⁻²), the power conversion efficiency η isdefined as

$\eta = \frac{V_{oc}J_{sc}{FF}}{P_{solar}}$

It has been reported by Sargent, E., in “Infrared photovoltaics made bysolution processing,” Nat. Photonics 3, 325-331 (2009), and Hillhouse H.S., et al., in “Solar cells from colloidal nanocrystals: Fundamentals,materials, devices, and economics” Curr. Opin. Colloid Interface Sci.14, 245-259 (2009), that the use of colloidal quantum dots as thelight-harvesting material provides photovoltaic cells with high powerconversion efficiencies. Colloidal quantum dot photovoltaics offer boththe ability to form the light-harvesting layer by solution processingand the ability to tune the bandgap over a wide range, benefits that areavailable in both single-junction and multijunction cells. The abilityto tune the bandgap also enables the use of inexpensive, abundantultralow-bandgap semiconductors that are otherwise unsuitable forphotovoltaic energy conversion. By combining lead chalcogenide quantumdots and Schottky junctions, photovoltaic cells with efficiencies of3.4% have been achieved, as reported by Ma, W., et al., “Photovoltaicdevices employing ternary PbS_(x)Se_(1−x) Nanocrystals,” Nano Lett. 9,1699-1703 (2009), and others. Significant progress has also beenachieved by sensitizing nanoporous TiO₂ electrodes with a thin layer ofcolloidal quantum dots, with power conversion efficiencies of 3.2%. Seefor example Fan, S., et al., “Highly efficient CdSequantum-dot-sensitized TiO₂ photoelectrodes for solar cellapplications,” Electrochem. Commun. 11, 1337-1330 (2009).

Both colloidal quantum dots and Schottky devices pose certainlimitations on photovoltaic efficiencies, however. In Schottky devices,both the V_(oc) and the FF values have fallen well below theirpotential, and in cells sensitized by colloidal quantum dots, the J_(sc)values are generally lower despite the increases in V_(oc) and FF.

SUMMARY OF THE INVENTION

It has now been discovered that the limitations of colloidal quantum dotphotovoltaics as noted above can be significantly reduced or overcome bythe pairing of a layer of light-harvesting nanoparticles with a layer ofan electron-accepting material such that the junction between theselayers is substantially depleted of both free electrons and free holeson at least one side of the junction when the device is not illuminated.An effective means of achieving this depletion is by selecting materialsfor these two layers that are of different bandgap magnitudes. Such ajunction is thus a heterojunction by virtue of the two differentmaterials on either side of the junction, and in particular a depletedheterojunction by virtue of the low level or absence of both freeelectrons and free holes in the vicinity of the junction. The depletionarises from charge transfer from the electron-accepting contact to theto the nanoparticles. In certain embodiments of the invention, thenanoparticles are quantum dots, include p-type colloidal quantum dots,and the electron-accepting layer is, or includes, a metal oxide. Thedepletion is at least partly attributable to a relatively low chargedensity in the electron-accepting layer, as compared to that of themetal contact of a Schottky junction, which has a very high freeelectron density.

Particular embodiments of photovoltaic devices within the scope of thisinvention offer further advantages over photovoltaic devices of theprior art. For example, the use of a metal oxide as theelectron-accepting layer allows the device to be configured with theelectron-accepting layer as the front surface of the device or as thelayer that the solar rays first penetrate upon entering the twosemiconductor layers that form the photovoltaic junction. The electronsliberated by the rays are thus less susceptible to recombination withthe holes since the electrons in these embodiments have a shorterdistance to travel before reaching their destination electrode. Also, inembodiments in which the junction is that between a metal oxide andquantum dots, the junction is better defined and easier to passivate,and thus less susceptible to defects, than a metal-semiconductorSchottky junction. This avoids the occurrence of Fermi-level pinning atthe interface. Still further, these embodiments present less of abarrier to hole injection than a Schottky device by introducing a largediscontinuity in the valence band and by minimizing the electron densityat the interface.

It has further been discovered that the performance of nanocrystals inphotovoltaic devices and in optoelectronic devices in general, andparticularly nanocrystals with surface anions, is enhanced by depositingcations over the nanocrystals to form a first or inner shell anddeposing anions over the first shell to form a second or outer shell.The inner and outer shells together passivate surface defects on thenanocrystal which tend to disrupt the quantum confinement of thenanocrystal. Passivation is known to be achievable by the placement ofshort organic ligands such as ethanedithiol, butylamine, ormercaptopropionic acid on the nanocrystal surface. The use of cation andanion shells in place of these ligands offers the advantages that thecation shells bind to the anions on the nanocrystal surface rather thanto the cations, as organic ligands tend to do, and the ionic bonds arestable upon exposure to air and light, and particularly moisture,oxygen, and heat. Further advantages of these cation and anion shellsare that by avoiding the need for organic ligands, these shells allowthe nanocrystals to reside very close to each other in thelight-absorbing film and thereby promote electron wave function overlapand carrier mobility, valuable features that are typically impeded byorganic ligands.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plot of current density vs. voltage for examples of depletedheterojunction photovoltaic cells within the scope of the presentinvention.

FIG. 2 is a plot of current vs. voltage for examples of depletedheterojunction photovoltaic cells within the scope of the presentinvention.

FIG. 3 is a plot of external quantum efficiency vs. wavelength forexamples of depleted heterojunction photovoltaic cells within the scopeof the present invention.

FIG. 4 is a plot of device capacitance vs. bias voltage and deviceresistance vs. bias voltage for examples of depleted heterojunctionphotovoltaic cells within the scope of the present invention.

FIG. 5 shows absorption spectra of examples of depleted heterojunctionphotovoltaic cells with dual-shell-passivated quantum dots within thescope of the present invention.

FIG. 6 is a plot of carrier lifetime vs. light intensity for examples ofdepleted heterojunction photovoltaic cells with dual-shell-passivatedquantum dots within the scope of the present invention.

FIG. 7 is a plot of current density vs. voltage for examples of depletedheterojunction photovoltaic cells with dual-shell-passivated quantumdots within the scope of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

The term “substantially depleted” as used herein to characterize theregion(s) adjacent to a heterojunction denotes that the charge densityin the region(s) is orders of magnitude less than that of the metal sideof a Schottky junction. In certain heterojunction regions of theinvention, the charge density is three or more orders of magnitude lessthan the charge density of conducting metals, and in many of these, thecharge density is four or more, five or more, or six or more orders ofmagnitude less. Particularly effective results can be achieved when thedepleted charge density is on the n-type electron accepting layer sideof the junction. In many embodiments of the invention, a range of chargedensity in the depleted region is about 1×10¹² cm⁻³ to about 1×10¹⁸cm⁻³, or alternatively about 1×10¹⁴ cm⁻³ to about 1×10¹⁷ cm⁻³, or as afurther alternative about 1×10¹⁵ cm⁻³ to about 1×10¹⁶ cm⁻³.

To achieve a depleted heterojunction by use of materials of differentbandgap magnitudes on the two sides of the junction, effective resultsin many cases can be achieved with a bandgap difference (i.e., thedifference between the bandgap magnitude on one side of the junction andthe bandgap magnitude on the other side of the junction) of at leastabout 1.5 eV, or within the range of from about 1.5 eV to about 5 eV, oreven more effectively within the range of from about 2 eV to about 5 eV.With an n-type electron-accepting layer on one side of the junction andp-type light-absorbing nanoparticles on the other, the bandgap ofgreater magnitude will reside in the n-type electron-accepting layer.

Quantum dots are particularly useful as the nanoparticles, and colloidalquantum dots, i.e., quantum dots manufactured by colloid chemistry, arenotable examples. Of these, metal chalcogenide quantum dots are wellknown in the art, and lead chalcogenide, and particularly lead sulfide,quantum dots are of particular interest. Quantum dots are known toabsorb light at wavelengths related to the diameters of individualquantum dots, and this property can be used in the present invention toselect or optimize the light-absorbing characteristics of the quantumdots. In many cases, quantum dots with a number-average diameter withinthe range of about 2 nm to about 15 nm can be used effectively, whilethose with a number-average diameter within the range of about 3 nm toabout 10 nm are often the most appropriate, and among these the range ofabout 3 nm to about 6 nm are often even more useful.

The n-type electron-accepting layer can vary widely in composition,provided that the combination of n-type electron-accepting layer andlight-absorbing nanoparticles when placed in contact form the depletedheterojunction described above. Metal oxides are examples of materialsthat can serve effectively as the n-type electron-accepting layer, and aparticularly useful example of a metal oxide is titanium dioxide.

In those aspects of the invention that relate to nanoparticles withinner passivating shells of cations and outer passivating shells ofanions, the core of such a nanoparticle is generally a quantum dothaving exposed anions at its surface. As noted above, the quantum dotcore is in many cases a metal chalcogenide colloidal quantum dot, mostoften a metal sulfide colloidal quantum dot. A noted example is leadsulfide, and lead sulfide quantum dots are often lead rich, with asurface composed primarily of exposed Pb²⁺ ions but also containingexposed S²⁻ ions. The cations of the inner shell bind to, and therebypassivate, the S²⁻ ions at the core surface, while the anions in theouter shell bind to, and thereby passivate, the cations of the innershell. Examples of cations that can be used for the first shell areCd²⁺, Pb²⁺, Zn²⁺, and Sn²⁺. Among these, Cd²⁺ is particularly convenientand effective. Examples of anions effective for use as the second shellare halogen ions and the thiocyanate ion. Of these, halogen ions, andparticularly bromine ion, are optimal or particularly convenient incertain cases. These dual-shelled nanoparticles are useful as thelight-absorbing nanoparticles of the depleted heterojunctions describedabove, but are also useful in optoelectronic devices in general, i.e.,any devices in which the particles serve to absorb light energy andconvert the absorbed energy to an electric current.

In further aspects, therefore, the present invention resides in theformation of passivated p-type semiconductor nanoparticles without usingorganic ligands as passivating agents. This is achieved by treatingp-type semiconductor quantum dots that have surface anions with asolution of a cation-containing reagent that passivates the surfaceanions, and then treating the resulting cation-treated quantum dot corewith a solution of a reagent that contains anions that passivate thecations. Noting the example of Cd²⁺ as a cation useful for thepassivation of the quantum dot core, an example of a Cd²⁺-containingreagent is cadmium(II) chloride-tetradecylphosphonic acid-oleylamine.Examples of anion-containing reagents are quaternary ammonium halidesand thiocyanates, and specific examples are cetyltrimethylammoniumbromide, hexatrimethylammonium chloride, tetrabutylammonium iodide, andtetrbutylammonium thiocyanate.

Photovoltaic devices utilizing one or more of the features describedabove will typically contain at least two electrodes, one electricallyconnected to each of the two semiconductor layers of the heterojunction.In a heterojunction between a n-type metal oxide layer and a layer ofp-type metal chalcogenide colloidal quantum dots, for example, a firstelectrode will be in direct electrical contact with the n-type metaloxide layer and a second electrode will be in contact with the colloidalquantum dot layer. The first electrode in many cases is alight-transmitting electrode, and examples are aluminum oxide, zincoxide, indium tin oxide (ITO), and fluorine-doped tin oxide (FTO). Thesecond electrode in many cases is either nickel, lithium fluoride,platinum, palladium, silver, gold, or copper, or alloys of two or moreof these metals, such as alloys of silver, gold, and copper. One exampleof a combination of electrode materials is fluorine-doped tin oxide asthe first electrode and gold as the second electrode.

EXAMPLE 1

This example illustrates the preparation of depleted heterojunctionphotovoltaic cells within the scope of the present invention, eachformed by depositing a layer of PbS colloidal quantum dots(approximately 10¹⁷ cm⁻³ n-type doping) of varying diameters—3.7 nm(bandgap 1.3 eV), 4.3 nm (bandgap 1.1 eV), and 5.5 nm (bandgap 0.9eV)—over transparent TiO₂ electrodes.

The TiO₂ electrodes were prepared on SnO₂:F (FTO)-coated glasssubstrates (Pilkington TEC 15, Hartford Glass, Inc., Hartford City,Ind., USA) with a TiO₂ paste (DSL-90T, Dyesol Ltd., Queanbeyan, NSW,Australia) as follows. The FTO substrates were first rinsed withtoluene, then sonicated for twenty minutes in a mixture of Triton inde-ionized water (1-3% by volume). Separately, a TiO₂ paste was preparedby combining one part by weight TiO₂ nanoparticles with three parts byweight terpineol. The paste was then spin cast at 1500 rpm for ninetyseconds on the TiCl₄-treated FTO substrates. One edge of each substratewas then wiped free of the paste with a swab soaked in isopropyl alcoholto expose a section of clean FTO for electrical contacting. This wasimmediately followed by sintering for one hour on a hotplate at 400° C.The substrates were then placed in a bath of 60 mM TiCl₄ in de-ionizedwater, and baked in the bath at 70° C. for thirty minutes. They werethen rinsed with de-ionized water, dried with nitrogen, and placed in a520° C. tube furnace for one hour, then cooled to room temperature. Thesample was then allowed to cool, and the TiCl₄ treatment was repeated,followed by a final heating to 520° C. The substrates were then placedin individual substrate holders and stored in air for up to one weekprior to further processing.

PbS colloidal quantum dots were prepared as follows.Bis(trimethylsilyl)sulphide (TMS, synthesis grade) (0.18g, 1 mol) wasadded to 1-octadecene (10 mL), which had been dried and degassed byheating to 80° C. under vacuum for 24 hours. A mixture of oleic acid(1.34 g, 4.8 mmol), PbO (0.45 g, 2.0 mmol), and 1-octadecene (14.2 g,56.2 mmol) was heated to 95° C. under vacuum for sixteen hours, thenplaced under Ar. The flask temperature was increased to 120° C. and theTMS/octadecene mixture was injected, causing the temperature to drop toabout 95° C., and the flask was allowed to cool to 36° C. Thenanocrystals were precipitated with 50 mL of distilled acetone andcentrifuged under ambient conditions. The supernatant was thendiscarded, and the precipitate was redispersed in toluene, precipitatedagain with 20 mL of acetone, centrifuged for five minutes, dried, andagain dispersed in toluene (about 200 mg mL⁻¹). The nanocrystals werethen placed in a N₂-filled glove box, where they were precipitated twicewith methane and then finally redispersed at 25 or 50 mg mL⁻¹ in octane.

The resulting oleate-capped PbS quantum dots were deposited on the TiO₂by multilayer spin-coating of the TiO₂ surface with 25 or 50 mg/mLsolutions of the quantum dots in octane under ambient conditions. Eachlayer was deposited at 2500 rpm, then treated briefly with 10%3-mercaptopropionic acid in methanol to displace the oleate ligand andthereby render the quantum dots insoluble, then rinsed with methanol andoctane. Fifteen deposition cycles using the 25 mg/mL dispersion producedthermally stable layers 22 nm in thickness on the TiO₂ substrate, andeight deposition cycles using the 50 mg/mL dispersion also producedthermally stable layers of the same thickness. Each layered medium wasthen transferred to a glove box with N₂ atmosphere and left overnight. Agold contact was then deposited on the quantum dot layer by DCsputtering with 5 m Torr Ar pressure at a power density of 1 W cm⁻²through a shadow mask to thicknesses of 150 nm to 200 nm.Spatially-resolved X-ray elemental analyses and transmission electronmicroscopy were performed on a thin section sample prepared byfocused-ion-beam milling, and revealed very little interpenetration ofthe quantum dot and TiO₂ layers.

FIG. 1 is a plot of the photovoltaic response of a depletedheterojunction solar cell as prepared above, expressed as currentdensity in mA cm⁻² vs. voltage, with the lower curve representing thedark current and the upper curve representing the illuminated current ofa cell fabricated with 1.3 eV-bandgap quantum dots (3.7 nm). The datawas measured using a Keithley 2400 source-meter under ambientconditions. The solar spectrum at AM1.5 was simulated to within class Aspecifications with a Xe lamp and filters with the intensity adjusted to100 mW cm⁻². The source intensity was measured with a Melles-Griotbroadband power meter (responsive from 300 nm to 2000 nm), through acircular 0.049cm ² aperture at the position of the sample and confirmedwith a calibrated solar cell. The accuracy of the power measurement wasestimated to be ±7%. For the five devices having 1.3 eV-bandgap quantumdots, the average value of V_(oc) was 0.53±0.02V, the average value ofJ_(sc) was 15.4±1.4 mA cm⁻², and the average value of FF was 57±4%. Theaverage AM1.5 power conversion efficiency η was thus 4.9±0.3%. For thehighest-performing device, V_(oc) was 0.52V, J_(sc) was 16.4 mA cm⁻²,and FF was 58%, yielding η of 5.1%.

FIG. 2 is a plot of the photovoltaic response of a depletedheterojunction solar cell as prepared above, expressed as current in mAvs. voltage, with the lower curve representing cells fabricated with 0.9eV-bandgap (5.5 nm) quantum dots, the middle curve representing thecells fabricated with 1.1 eV-bandgap (4.3 nm) quantum dots, and theupper curve representing the cells fabricated with 1.3 eV-bandgap (3.7nm) quantum dots. This figure shows that the largest quantum dots hadthe smallest driving force for electron transfer in TiO₂, and yetdevices with these 0.9 eV bandgaps still showed a short-circuit currentdensity J_(sc) above 10 mA/cm² and an open-circuit voltage V_(oc) of0.38V. This indicates that minimal band offset is required for efficientelectron transfer from the PbS colloidal quantum dots into the TiO₂electrode. This is in contrast to organic photovoltaics, which have alarge band offset between the electron donor and acceptor, the largeband offset imposing a substantial penalty on efficiency.

FIG. 3 is a plot of external quantum efficiency (EQE) vs. wavelength andof absorption vs. wavelength, with the lower curve representing the EQEfor the best-performing 1.3 eV-bandgap quantum dot device and the uppercurve representing the spectral absorption of the same device. The EQEis the ratio of extracted electrons to incident photons and the curve isalso known as the incident photon conversion efficiency spectrum. TheEQE was obtained by passing the output of a 400W Xe lamp through amonochromator and using appropriate order-sorting filters. Thecollimated output of the monochromator was measured through a 1.5 nmaperture with a calibrated Newport 818-UV power meter. The measurementbandwidth was about 40 nm and the intensity varied with the spectrum ofthe Xe lamp. The average intensity was 0.3mW cm⁻². The current-voltageresponse was measured with Keithley 2400 source-meters. The plot showsthat at short wavelengths, the EQE reached values above 60%, while atlonger wavelengths the EQE had a peak of 24%.

FIG. 4 is a plot of device capacitance vs. bias voltage and of deviceresistance vs. bias voltage. The capacitance arises from the depletionlayer due to charge transfer from TiO₂ to the PbS colloidal quantum dotlayer. Capacitance-voltage measurements were performed directly on thephotovoltaic devices using an Agilent 4284A LCR meter. Absorptionspectroscopy was performed on a Cary 500 UV-vis-IR Scanphotospectrometer. The impedance was acquired at 2 kHz with a signalamplitude of 10 mV, and is represented in FIG. 4 in terms of equivalentparallel resistance R_(p) and capacitance C_(p) for a device with acontact area of 0.03 cm³. The plot shows that the capacitance, and itsassociated depletion layer distributed between the two semiconductors,persist up to a bias of 0.6V, close to the observed open-circuit voltagevalue. This is a direct indication of the presence of a built-in fieldthat efficiently drives the separation of photogenerated carriers.

EXAMPLE 2

This example illustrates the preparation and use of nanoparticlescontaining a quantum dot core, an inner shell of cations and an outershell of anions, within the scope of the present invention.

Colloidal quantum dots capped with oleic acid ligands were synthesizedand stripped of their oleate ligands, in the manner described inExample 1. These quantum dots were prepared with an excess of Pb duringsynthesis, resulting in a lead-rich bulk composition but with sulfuratoms on their surfaces, either from nonpolar {100} and {110} or polar{111} facets in their crystal structure. To form the inner shell of Cdcations over these PbS cores, the nanoparticles were treated with asolution of CdCl₂-tetradecylphosphonic acid-oleylamine (CdCl₂-TDPA-OLA).This treatment resulted in a slight redshift (between 6 and 24 nm) ofthe excitonic absorption, suggesting growth of a partial monolayer ofhighly cation-rich material on the surface, an interpretation reinforcedby the approximately 30 nm blueshift observed when a control treatmentinvolving TDPA-OLA only (no CdCl₂) was implemented. Elemental analysisand X-ray photoelectron microscopy both indicated 0.3% atomic ratio ofcadmium to other elements present in powders of the resultant samples.X-ray diffraction indicated that no purely Cd-based phase (such as CdS)was present.

An outer shell of bromine ions was then applied by the use of a solutionof cetyltrimethylammonium bromide in methanol. Thecetyltrimethylammonium cations combined with any remaining oleates onthe particles to form salts that were then removed with a final methanolrinse. The cetyltrimethylammonium bromide treatment and methanol rinsewere conducted in air at room temperature (23° C.), including theabsence of hydrazine. The absence of any appreciable amounts of organicsat the outer surfaces of the treated particles was confirmed by FTIRspectra showing a complete absence of C-H vibrations at 2922 cm⁻¹ and2852 cm⁻¹. The presence of a significant amount of bromide in the outerapplied film was confirmed by X-ray photoelectron spectroscopy (XPS) andenergy-dispersive X-ray spectroscopy (EDX), and simple calculationindicated an approximate 1:1 ratio of bromide ions to surface cations.Elemental analysis confirmed that the 0.3% of Cd cations presentfollowing the initial CdCl₂-TDPA-OLA were still present afterapplication of the bromide shell.

Photovoltaic devices utilizing these dual-shell-passivated quantum dotswere fabricated in the same manner as described in Example 1 above. Ascanning electron micrograph showed that the quantum dot layer wasapproximately 300 nm in thickness and was free of the voids and cracksthat often occur in films made from layer-by-layer deposition.Absorption spectra of the devices were obtained in a double pass byincluding reflection from the Au top contact. The spectra of devicesmade using 9, 11, and 13 quantum dot layers are shown in FIG. 5, whichalso includes corresponding spectra from the bare FTO/TiO₂ substrate.The absorption peak at 950 nm is the excitonic peak of the PbS quantumdots. This indicates that quantum confinement of the core quantum dotswas preserved in the shelled form. A reduction in interparticle distanceis suggested by the red-shift (−100 meV) of the excitonic peak in thefinal film as compared to the excitonic peak of dots in solution. Uponexposure to 100 mW/cm² solar illumination, the device showed an opencircuit voltage (V_(oc)) of 0.45V, a short-circuit current density(J_(sc)) of 21.8 mA/cm², and a fill factor (FF) of 59%, yielding a powerconversion efficiency η of 5.76%. Integration of the net absorption ofthe quantum dot film over the AM1.5G spectrum indicates that a filmhaving 100% quantum efficiency would have achieved a circuit currentdensity (J_(sc)) of 24.4 mA/cm². Comparing this with the measuredcircuit current density (J_(sc)) of 21.8 mA/cm² indicates that theinternal quantum efficiency (IQE) averaged across the entire broadbandabsorbing region of 400-1150 nm exceeds 90%, indicating minimalrecombination loss and efficient carrier extraction.

The doping density and carrier lifetime of the dual-shell-passivatedquantum dot films were determined by capacitance-voltage (C-V) andV_(oc) decay analyses, respectively. The C-V analysis showed that dopingwas a full order of magnitude lower than in the lowest-doped organicligand PbS and PbSe quantum dot films, and the carrier lifetime τ, whichis shown in FIG. 6, was approximately twice as long as that of a controldevice made using a bidentate organic ligand (3-mercaptopropionic acid,also shown in FIG. 6), reaching a remarkably long lifetime of over 40μsec even under full solar 100 mW/cm² illumination.

The dual-shell-passivated quantum dots also demonstrated an improvedresistance to oxidation. FIG. 7 is a plot of current density vs.voltage, comparing a layer of dual-shell-passivated quantum dots inaccordance with the invention with quantum dots bearing3-mercaptopropionic acid ligands, each shown both fresh (immediatelyafter fabrication) and after ten days of storage under ambientconditions on a laboratory bench. The dual-shell-passivated quantum dotsshowed no significant change in performance over the ten-day period,while the organic ligand-capped quantum dots underwent a complete lossof efficiency over the same period.

To demonstrate the effectiveness of inner shells of anions other thanbromide ions, devices were made containing dual-shell-passivated quantumdots with a variety of anions, and measurements of the photovoltaicperformance characteristics were made. The anion-bearing reagents werehexatrimethylammonium chloride (HTAC), cetyltrimethylammonium bromide(CTAB), tetrabutylammonium iodide (TBAI), and tetrabutylammoniumthiocyanate (TBAT). The parameters measured were J_(sc) in mA/cm²,V_(oc) in V, FF in %, η in %, shunt resistance R_(sh) and seriesresistance R_(s), and rectification (the current between forward bias+1V and reverse bias −1V), and are listed in the following Table.

Recti- Reagent Ligand J_(sc) V_(oc) FF η R_(sh) R_(s) fication HTAC Cl⁻17.1 0.43 55% 4.08% 3388 1112 10391 CTAB Br⁻ 21.8 0.35 59% 5.76% 3351 602920 TBAI I⁻ 20.2 0.43 43% 3.76% 3195 148 4916 TBAT SCN⁻ 13.9 0.43 30%1.72% 1924 847 4232

In the claims appended hereto, the term “a” or “an” is intended to mean“one or more.” The term “comprise” and variations thereof such as“comprises” and “comprising,” when preceding the recitation of a step oran element, are intended to mean that the addition of further steps orelements is optional and not excluded. All patents, patent applications,and other published reference materials cited in this specification arehereby incorporated herein by reference in their entirety. Anydiscrepancy between any reference material cited herein or any prior artin general and an explicit teaching of this specification is intended tobe resolved in favor of the teaching in this specification. Thisincludes any discrepancy between an art-understood definition of a wordor phrase and a definition explicitly provided in this specification ofthe same word or phrase.

What is claimed is:
 1. A nanoparticle comprising: a quantum dot corewith surface anions; an inner shell comprising cations that are bound tothe surface anions of the quantum dot core; and an outer shellcomprising anions that are bound to the cations of the inner shell. 2.The nanoparticle of claim 1, wherein the quantum dot core is a metalchalcogenide colloidal quantum dot.
 3. The nanoparticle of claim 1,wherein the quantum dot core is a metal sulfide colloidal quantum dot.4. The nanoparticle of claim 1, wherein the quantum dot core is a metalsulfide colloidal quantum dot and the cations of the inner shell are amember selected from the group consisting of Cd²⁺, Pb²⁺, Zn²⁺, and Sn²⁺.5. The nanoparticle of claim 1, wherein the quantum dot core is a metalsulfide colloidal quantum dot and the cations of the inner shell areCd²⁺.
 6. The nanoparticle of claim 1, wherein the quantum dot core is ametal sulfide colloidal quantum dot and the cations of the inner shellare Cd²⁺ and the anions of the outer shell are a member selected fromthe group consisting of halogen ions and thiocyanate ion.
 7. Thenanoparticle of claim 1, wherein the quantum dot core is a metal sulfidecolloidal quantum dot, the cations of the inner shell are Cd²⁺, and theanions of the outer shell are halogen ions.
 8. The nanoparticle of claim1, wherein the quantum dot core is a metal sulfide colloidal quantumdot, the cations of the inner shell are Cd²⁺, and the anions of theouter shell are bromine ions.
 9. A photoelectric device comprising: alight-transmitting first electrode; a first semiconductor layer indirect electrical contact with the first electrode; a secondsemiconductor layer in direct electrical contact with the firstsemiconductor layer and comprising light-absorbing nanoparticles,wherein each of the nanoparticles comprises a quantum dot core, a firstshell comprising cations bound to the quantum dot core, and a secondshell comprising anions bound to the cations; and a second electrode indirect electrical contact with the second semiconductor layer.
 10. Thephotoelectric device of claim 9, wherein the quantum dot core is a metalsulfide colloidal quantum dot and the cations of the first shell areCd²⁺.
 11. The photoelectric device of claim 9, wherein the cations ofthe first shell are Cd²⁺ and the anions of the second shell are a memberselected from the group consisting of halogen ions and thiocyanate ion.12. The photoelectric device of claim 9, wherein the quantum dot core isa metal sulfide colloidal quantum dot, the cations of the first shellare Cd²⁺, and the anions of the second shell are halogen ions.
 13. Thephotoelectric device of claim 9, wherein the quantum dot core is a metalsulfide colloidal quantum dot, the cations of the first shell are Cd²⁺,and the anions of the second shell are bromine ions.
 14. Thephotoelectric device of claim 9, wherein the first semiconductor layerand the nanoparticles form a photovoltaic junction therebetween, atleast one side of which is substantially depleted of free electrons andof free holes when the device is not illuminated.
 15. A method offorming passivated p-type semiconductor nanoparticles, the methodcomprising: treating p-type semiconductor colloidal quantum dots havingsurface anions with a solution of a cation-containing reagent thatpassivates the surface anions to form colloidal quantum dots coated withthe cations, and treating the colloidal quantum dots with a solution ofan anion-containing reagent that passivates the cations.
 16. The methodof claim 15, wherein the p-type semiconductor colloidal quantum dots aremetal chalcogenide colloidal quantum dots, metal sulfide colloidalquantum dots, or lead sulfide colloidal quantum dots.
 17. The method ofclaim 15, wherein the cation of the cation-containing reagent is amember selected from the group consisting of Cd²⁺, Pb²⁺, Zn²⁺, and Sn²⁺.18. The method of claim 15, wherein the anion of the anion-containingreagent is a member selected from the group consisting of halogen ionsand thiocyanate ion.
 19. The method of claim 15, wherein the anion ofthe anion-containing reagent is a bromine ion.
 20. The method of claim15, wherein the anion-containing reagent is a member selected from thegroup consisting of cetyltrimethylammonium bromide,hexatrimethylammonium chloride, tetrabutylammonium iodide, andtetrabutylammonium thiocyanate.